In essence a dual-clutch transmission consists of two sections or partial transmissions with various gearwheel pairs, two powershiftable clutches each associated with one of the partial transmissions, a drive input shaft and a drive output shaft and, depending on the number of gears, interlocking and unsynchronized shifting clutches. The gears are arranged in alternation in the two partial transmissions so that a gear can be preselected in the load-free partial transmission while the torque is being transmitted by the other partial transmission.
A gearshift is carried out by transferring the torque to be transmitted from one powershiftable clutch to the other, so that the gears can be shifted while free from load and without traction force interruption and without tensioning the shiftable gears. As a rule the odd-numbered gears can be engaged in one partial transmission and the even-numbered ones and the reverse gear in the other partial transmission.
The advantages of dual-clutch transmissions include better efficiency in comparison with transmissions of planetary design, a freer choice of gear ratios by virtue of the partial transmissions, ability to operate at high rotation speeds, and sporty driving behavior.
From the prior art dual-clutch transmissions of coaxial design are known, which comprise a main transmission and downstream therefrom a group transmission, wherein as a rule powershifts cannot be carried out which entail a group change. Disadvantageously, a non-powershifted group change takes a long time since a switchover in the group transmission and a shift in the main transmission have to be carried out, which results in a long traction force interruption.
DE 10 2005 044 068 A1 describes a powershiftable group transmission with a dual clutch, which comprises a splitter group, a direct gear, a main group and a hollow shaft arranged on a countershaft. In this case it is possible to shift, without traction force interruption, between gears produced by a change of the input constants within the splitter group of the transmission without changing the gear step in the main group. Furthermore it is possible to shift, without traction force interruption, between two adjacent gears produced by a gear ratio change within the main group. In this known transmission a power path can pass from the first individual clutch of the dual clutch, via a direct gear, to a main shaft, whereas the two forward gears immediately adjacent to the direct gear pass by way of the second individual clutch of the dual clutch and the countershaft and the hollow shaft arranged concentrically with the countershaft carries at least two gearwheels, of which one gearwheel belongs to the second input constant whereas a gearwheel adjacent to the dual clutch belongs to the first input constant. Moreover, the power path of a forward gear passes from the second individual clutch by way of the first input constant, the second input constant and the main shaft coupled in a rotationally fixed manner to a further gearwheel of the second input constant.
DE 10 2010 041 410 A1 by the present applicant describes a dual-clutch transmission comprising two clutches, whose input sides are connected to a drive input shaft and whose output sides are connected respectively to one of two transmission input shafts that are arranged coaxially with one another, with at least two countershafts arranged coaxially with one another and with a drive output shaft arranged coaxially with the drive input shaft, wherein a number of gear wheel planes are provided with which several shifting elements are associated for the engagement of loose gear wheels and/or for forming connections to shafts, in such manner that a number of forward gears can be engaged, the number of powershiftable forward gears exceeding the number of gear wheel planes by at least two.
From WO/2011/069526 a dual-clutch transmission is known, which comprises a main transmission and a group transmission connected downstream therefrom, in which, in a gear the main transmission transmits the power from an input shaft to the main shaft, while the group transmission is shifted to “low”. In this case the gear is produced by means of the gear ratio of the group transmission. In the next-higher gear the group transmission is not connected into the power path and can therefore be shifted to “high”, so that the transition to further, higher gears can be powershifted.